Research, development, design, construction, manufacture, marketing
and customer support will be increasingly integrated so that they would
work together as a single component virtually joining the clients,
designers and manufacturers of automotive components. In this respect,
the European Commission, in collaboration with other important consortia
of steel companies implemented a number of projects in recent years:
Ultra-Light Steel Auto Body--ULSAB, Ultra-Light Steel Auto
Closures--ULSAC, ULSAB-AVC, FSV BEV and SuperLIGHT-CAR, The key
objective was to reduce CO2 emissions and mitigate the climate changes.
Requirements relating to reducing emissions and mitigating climate
changes in the production and operation of the vehicles are required to
reconcile with the requirements of passengers and pedestrians safety as
well as power, legislative as well as designer ones (Evin et al., 2012).

[FIGURE 1 OMITTED]

The surviving of passengers (passenger safety) in an accident is
determined by the size of the human body congestion and the
occupant's survival space--Fig. 1. Deformation work for plastic
deformation of deformation zone components in the engine compartment and
trunk must be consumed during crash for absorption of the impact kinetic
energy. Thus, the larger the deformation work of components in the area
of trunk and engine is, the less overloading of passengers occurs from
the moment of contact of stronger and stiffer components in the front
and the rear auto body part with a fixed barrier (Evin, 2011). Stronger
and stiffer components in the area of cab must prevent the penetration
of auto body components into passenger compartment (cab) during a crash.
When designing the SuperLIGHT-CAR concepts, the components of
deformation zones in the area of engine and trunk were made mostly of DP
steels--Dual Phase, TRIP steels--Transformation Induced Plasticity,
TWIP--Twinning Induced Plasticity, ASS--austenitic steels. Components in
cabin space (in the passengers zone) were made of ultra-high strength
steels (UHSS) with yield strength higher than 550 MPa (MART martensitic,
FB ferritic-bainitic steels, TWIP steel--Twinning Induced Plasticity,
CP-Complex Phase steel, hot-formed boron steels--formed hot, bored,
steel heat-treated after forming--post forming heat treated) as well as
TRIP, TWIP and austenitic steels with a certain degree of predeformation
(e.g. hydromechanical forming). There were also used HSS steels with
yield strength from 210 to 550 MPa and an tensile strength Rm from 270
to 700 MPa (HSIF--High-Strength Interstitial Free, HSLA--High Strength
Low-Alloy, micro-alloyed with BH effect, carbon-manganese sheets),
stampings and castings made of aluminium and magnesium alloys as well as
composites (Evin et al., 2012; Hofmann, 2008; Rosenberg et al., 2009;
Kleiner et al., 2003; Aksoy et al., 1996; Takahashi, 2003). Material
composition of the SuperLIGHT-CAR auto body components allowed reaching
the body weight reduction of 74 kg (27%) and 115 kg (38%).

2. Application Aspects of AHSS

The combination of high strength and ductility that provide modern
AHSS can allow thinner components to be used in the cars construction
and also to improve the safeness due to their high energy-absorption
capabilities. The better formability of AHSS, compared to conventional
high strength steels of comparable strength give the automobile designer
a high degree of flexibility to optimize the component geometry. other
component performance criteria comprise stiffness, durability, crash
energy management (Evin, 2011).

[FIGURE 2 OMITTED]

The primary types of loading (longitudinal loading tension and
compression, bending, torsion, combined bending and torsion, shear
loading,) components of the body at impact are shown in Fig. 2. For the
longitudinal tensile or compressive force strength and deformation work
criteria given in (Rosenberg et al., 2009) can be used to predict the
stiffness.

The stiffness of a component is affected by material properties
(module of elasticity--E, yield stress--YS = [[sigma].sub.0.2%] true
yield stress--Y[S.sub.true] or true flow stresses-- [[sigma].sub.0.05],
[[sigma].sub.0.1]) as well as its geometry. The stiffness can be
predicted using the following relationship:

STF = [V.sub.0][YS/x.E] (1)

or by elastic work:

STFW = [V.sub.0][Y[S.sup.2]/[x.sup.2].E] (2)

The module of elasticity is constant for steel; considering eq. (2)
it means change the steel grade does affect the stiffness due to the
yield stress change. Therefore, to improve stiffness for constant
component geometry the material with higher yield stress must be
changed. The yield stress can be predicted by Hall Petch relationship as
the additive effect of the various mechanisms of hardening (Kuziak,
2008, Dzupon et al., 2007):

where d--the ferritic grain, or diameter of cells of dislocation
martensite,

[k.sub.y]--the characteristic of a barrier of grain boundaries
against dislocation movement,

[[sigma].sub.0]--stress required for movement of dislocations in
crystalographical lattice,

[DELTA][[sigma].sub.PR]--contribution of hardening by perlite,

[DELTA][[sigma].sub.D]--contribution of dislocation hardening,

[DELTA][[sigma].sub.S]--contribution of substitutional hardening,

[DELTA][[sigma].sub.IN]--contribution of interstitial hardening,

[DELTA][[sigma].sub.P]--contribution of precipitation hardening,

[DELTA][[sigma].sub.f]--contribution of phase hardening.

The yield strength increases in two ways: about BH effect (approx.
40 / 60 MPa) due to thermo-mechanical processing when the paint is baked
and about WH effect as a result of deformations--see Fig. 3. AHSS also
have good bake hardening ability (BH effect) and work hardening ability
(WH effect)--Fig. 4, then the true value of the yield stress can be:

Y[S.sub.true] = YS + BH + WH (4)

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

To evaluate the true flow stresses of different steel sheets, the
following Hollomon equation can be used:

[sigma] = K.[[epsilon].sup.n] (5)

and WH effect

WH = K.[[epsilon].sup.n] - YS (6)

where YS or Y[S.sub.0.2%]--yield stress at static tensile test,

BH--bake hardening effect (interstitial hardening),

WH--work hardening effect.

UTS--ultimate tensile strength,

[[epsilon].sub.r] or UE--uniform (homogenous) deformation,

n--strain hardening exponent,

K--strength coefficient

X--degree of safety (x = 1.6 / 2).

The strength of a component depends on its geometry and yield
and/or tensile strength--Fig. 5.

ST = YS/x (7)

Or

S[T.sub.true] = Y[S.sub.true]/x (8)

AHSS provide an advantage in the design flexibility over
conventional high strength steels due to their higher formability and
work hardening characteristics. These grades also have good bake
hardening ability--BH. Therefore, it is important to account for this
strength increase during the design process of car components in order
to avoid the over design that may occurs when the design process is
based upon as rolled mechanical properties specification. Both these
features enable achieving high strength of as-manufactured components.

The crashworthiness is an important characteristic that is
currently becoming increasingly important. Recent trends require for a
material to absorb more energy in crash scenario. The potential
absorption energy can be assessed based upon the area under the
stress-strain curves.

W = [YS + UTS/2].[[epsilon].sub.r] (9)

Better performance in crash of AHSS compared to classical high
strength steels is associated with higher work hardening rate and high
flow stress. This feature accounts for a more uniform strain
distribution in components in the crash event. Both, work hardening (WH)
and bake hardening (BH) significantly improve the energy absorption
characteristics due to the flow stress increase. Then the strain work
(Fig. 5) can be calculated according to equation (10):

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (10)

The fatigue properties of structural components depend on geometry,
thickness, applied loads and material endurance limit. Thus, high
strength combined with superior work hardening and bake hardening,
resulting in a significant increase in the as manufactured strength of
AHSS components, also results in a better fatigue resistance.

AHSS which fulfil these requirements include dual-phase
ferritic-martensitic (FM) steels. Microstructure of dual phase steels is
composed of soft ferrite matrix and 10-20% of hard martensite or
martensite-austenite (M-A) particles. This type of microstructure allows
achieving the yield strength Re in the range of 300 / 500 MPa and the
ultimate tensile strength in the range of 500 / 1200 MPa. When the
volume fraction of martensite exceeds 20%, DP steels are often called
partial martensitic. For some applications, also baintic constituent may
be desirable in the DP steel microstructure (Uthaisangsuk, 2008; Podder,
2007).

[FIGURE 5 OMITTED]

The contributions of hardening mechanisms in the martensitic
structure include the solid solution substitution element hardening, the
precipitation hardening, the primary austenitic grain size hardening and
the martensite morphology hardening. The dominant hardening effect of
martensite in dual phase steels is the carbon concentration in
martensite. It is relatively difficult to formulate regression equations
for the contributions of individual hardening mechanisms in martensite
as it is possible for polygonal ferrite, since it is impossible to
separate individual hardening mechanisms in martensite (Kuziak, 2008).

3. Methods for Prediction of Safety and Technological Formability
Characteristics of Body Components from Steel Sheets

When analyse safety and formability characteristics of auto body
components from steel sheets, it is necessary to define the location and
type of failure on stamped part. Tears occur in consequence of tensile
stress in the area of curve--Fig. 6.

Area of failure may be divided on three parts (Hrivnak, A. &
Evin, E., 2004):

1. area of tension: [[epsilon].sub.2] < 0,

2. area of plane strain: [[epsilon].sub.2] = 0,

3. area of stretching: [[epsilon].sub.2] > 0.

[FIGURE 6 OMITTED]

When a car crashes as well as at the production of body components
the failures by pure uniaxial tension or biaxial tension occurs only in
rare cases. In the practise it is ineffective to develop the test method
for each shape of car's components from steel sheet blanks. The
more effective way shows us to compare deformation properties of steel
sheets and components made of steel sheets, based on results of standard
tests that model schemes of its loading at production and its
application.

Stress of material in the area of stretching ([[epsilon].sub.2]
> 0) can be modelled by tensile test, cross tensile test, Erichsen
test, bulge test, Marciniak test, Nakazima test, etc. Stress of material
in the area of deep drawing ([[epsilon].sub.2] < 0) can be modelled
by tensile test with the notch radius on the samples r = 2 mm test,
Fukui test, Engelhardt test, etc.

3.1 Experimental Procedure

Experimental research for evaluating the strength and energy
absorption and formability of sheets with higher strength properties was
carried out on steel sheets of F-M produced by intercritical annealing
(specimens designated A1, A2, A3, A4, B1, B2) and specimens produced by
the method of controlled rolling (specimens denoted as C1, C2, C3, C4,
C5). The volume proportion of the individual structural components and
the ferrite grain size are shown in Table 1. Metallographic analysis of
the materials A and B show that they have a fine-grained
ferrite-martensite structure with martensite dispersion excluded in the
form of small islands which form mainly in the area of the ferrite grain
boundaries (Fig. 7). In the material C martensite formed large islands
and ferrite and martensite grains 'alternated' (Fig. 8) (Evin,
2011, Hrivnak, A. & Evin, E., 2004).

The materials C had a dual-phase structure. In many cases the
second phase showed a morphological feature of martensite or a mixed
nonpolyhedral structure. Based on the brief analysis of the
metallographic structure it may be concluded that a large difference was
detected in the morphology on distribution of martensite in the
materials A and B produced by intercritical annealing in comparison with
the materials C produced by controlled rolling.

Stress of material in the area of stretching ([[epsilon].sub.2]
> 0) was modelled by Erichsen test. Stretchability is expressed as IE
height of cup. Stress of material in the area of deep drawing
([[epsilon].sub.2] < 0) was modelled by cup test. Drawability is
expressed as the limiting draw ratio as follows:

Technological characteristics obtained by Erichsen test and cup
test as well as these values calculated for selected materials by
numerical simulation are shown in Table 3.

The numerical simulation of Erichsen test and cup test for selected
materials were realised in order to compare experimental and calculated
values. Based on tools dimensions used in experiments virtual CAD models
were created as it is shown in Fig. 9 for Erichsen test and Fig. 10 for
cup test.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

The numerical simulation of tests was done using Pam Stamp 2G
simulation software. Simulation models were meshed, positioned and
set-up in pre-processing module of the software, based on CAD data. To
define material models, yield law and anisotropy type following input
data were defined in Pam Stamp 2G preprocessor:

--basic material data (density, Young's modulus,
Poisson's constant),

--blank thickness,

--strain-hardening curve defined by Hollomon's law according
to data shown in Tab. 3--constant K and strain hardening exponent n,

--plastic strain ratio r as definition of sheet normal anisotropy,

--rolling direction 0[degrees] in x-axis of blanks,

--Yield law defined by Hill 48 model.

Note, the materials were considered here as isotropic so the planar
anisotropy of plastic strain ratio wasn't considered.

The results of numerical simulations were evaluated in
postprocessing module of Pam Stamp 2G simulation software. The maximum
forces and force dependencies were filtered by MVA filter with the range
of 25 due to its course oscillation given by numerical simulation--Fig.
11. Based on the finding the maximum drawing force, the IE height of cup
in Erichsen test was measured as well as the LDR in cup test was
calculated. The value of FLD0 was calculated by the software using
AutoKeeler mode because of the FLC curves for these materials
weren't experimentally measured. The results of height of cup IE,
LDR and FLD0 reached by numerical simulation and compared to
experimental ones are shown in Table 3. LDR values were determined from
the drawing forces (F draw) and the breaking force (F break) required to
fracture the wall of drawn part--Fig. 12.

[FIGURE 12 OMITTED]

4. Discussion of Obtained Results

Based on designers' experiences it is possible to define the
requirements for materials from the viewpoint of static strength and
energy absorption reliability (Evin, 2011). Effectiveness of static
strength (Fig. 14) is calculated as follows:

Comparison of the mechanical properties specified in the material
of the sheets of the material DC 04 with the measured values obtained
for the examined materials of the F-M steels (Table 2) show that the
yield strength (Re = 299-495 MPa) and the tensile strength (Rm = 593-792
MPa) of all materials was higher that of a mild steel DC 04.
Approximately the same volume fraction of martensite in the structure
the materials produced by intercritical annealing had lower yield limit
values than the materials produced by controlled rolling. The elongation
values ([A.sub.50] = 15-31%) of specimens A, B and C varied in the range
of materials suitable for slight drawing or bending and for other
materials in the range of materials unsuitable for deep-drawing. As in
the case of strength, the deformation properties values showed no large
difference between the materials produced by intercritical annealing and
the materials produced by controlled rolling.

Calculated values of the effectiveness of static strength and
energy absorption according to equation (12), (13), (14), (16) for high
strength dual phase steels has been compared to the steel sheets DC
04--Fig. 13 and Fig. 14. These results indicate the potential for weight
reduction from 42 to 135% with equivalent energy absorption. As it was
mentioned the most of the inner supporting construction elements of car
body are made of steel sheets. These elements are produced by operations
of bending, stretching and deep drawing. During bending deformation
hardening occurs only in small part of bend (in local deformation) of
stamped part, in non-deformed parts (in straight parts of stamped part)
deformation strain hardening doesn't occur. Stamped parts produced
by bending show non-homogenous distribution of deformation. During
deep-drawing and stretching operations of the stamped parts deformation
as well as deformation strain hardening occurs on whole area. The
deformation distributed at stretching is more homogenously than at deep
drawing operations. it is required to calculate with strain hardening
but also with interstitial hardening (BH effect- increasing the strength
about approximately 30 to 60 MPa) to optimize the material selection,
according to Eq. (4).

[FIGURE 13 OMITTED]

The exponent of strain hardening of the material react very
sensitively to the change in the condition of the structure and
substructure of the material and enable the limit of the loss of plastic
stability, reduction area, to be expressed more accurately. Up to this
limit there is a guarantee that plastic deformation doesn't
localize and there is no subsequent failure of the material. Then
effectiveness static strength by 5% degree of deformation can be
calculated according to equation:

EST = [K.[[epsilon].sup.n]/K.[[epsilon].sup.n.sub.DC04]]. 100[%]
(14)

[FIGURE 14 OMITTED]

Comparison of the constant K specified in the material of the
sheets of the material DC 04 with the measured values obtained for the
examined materials of the F-M steels (Table 2) show that the constant K
(K = 1052 - 1336 MPa) of all materials was higher that of a steel sheets
DC 04 and the values of the strain hardening exponent of materials
produced by intercritical annealing were greater or comparable with the
DC 04. Materials produced by rolling have shown lower values of strain
hardening exponent as DC 04. Approximately the same volume fraction of
martensite in the structure of materials produced by intercritical
annealing had higher strain hardening exponent and constant K values
than the materials produced by controlled rolling. The results confirmed
the interaction effect of ferrite and martensite reflected in an
increase of dislocation density in ferrite and at the ferrite-martensite
boundary and in an increase in flow stress. However, at assumption that
at production of certain stamped part 5% ([epsilon] = 0.05) deformation
and true stress is expressed by relation (4), dual phases materials
shows approximately from 100 to 200% higher strength as reference
material DC 04--Fig. 13.

Dual phase-steels exhibit of strain hardening effect, i.e. sustain
higher stresses at increased deformation. This effect corresponds to
increase in load car crash to the reference material. Then the strain
work (Fig. 15) can be calculated according to equation:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (15)

and effectiveness of energy absorption by 5% degree of deformation

EEA = [W/W[D.sub.C04]. 100[%] (16)

[FIGURE 15 OMITTED]

Based on the test results of technological forrnability it is
possible to compare the forrnability of dual phase steel sheets from the
viewpoint the forrnability of conventional low-carbon steel sheets. The
classification of conventional low-carbon steel sheets suitable for deep
drawing is given in Table 4.

[FIGURE 16 OMITTED]

The elongation values ([A.sub.50] = 15 / 31%) of specimens A, B and
C varied in the range of materials suitable for slight drawing or
bending and for other materials in the range of materials unsuitable for
deep-drawing. As in the case of strength, the deformation properties
values showed no large difference between the materials produced by
intercritical annealing and the materials produced by controlled
rolling. Deformation properties of dual-phase steels (tensibility,
uniform elongation UE, strain-hardening exponent) depend on the volume
fraction of martensite--Fig. 16.

Innovation tendencies in automotive industry (decreasing of mass,
saving of energy, ecology) lead to the use of high-strength steels of
new conceptions (micro-alloyed, bake hardening--BH, interstitial free
-IF, dual phase--DP, with transformation induced plasticity--TRIP). Even
though they show higher values of elongation, normal anisotropy
coefficient and exponent of strain hardening indicate the good
formability. High-strength steel sheets with tensile strength in the
range from 400 MPa to 800 MPa cannot be classified according to
conventional schemes of evaluation of formability because these steels
despite their higher strength show good formability (Hrivnak, A. &
Evin, E., 2004).

Suitability of dual phase steel sheets for deep drawing was
evaluated based on values recommended for qualitative grades of drawing
of classical steel sheets (deep drawing process--values LDR and
stretching--values IE and FLD0)--see Fig. 17 and Fig. 18. Values of
limiting ratio (LDR) for examined material evaluated by method of
intercritical annealing varied in the range from 2.068 to 2.096 and in
materials produced by controlled rolling from 1.93 to 1.97. We measured
higher values of the degrees of the LDR in approximately the same volume
fraction of martensite in structure of materials produced by controlled
rolling. The diagram LDR in Fig. 19 indicates that materials produced by
intercritical annealing appear to be suitable for deep drawing (DDQ)
whereas the materials produced by controlled rolling appear to be
suitable for drawing quality (DQ).

On the basis of value IE the materials A and G are suitable for
demanding operations of stretching, and materials A1 and A3 are suitable
for middle demanding operations of stretching--DSQ, and materials A2 and
C4 are suitable for lower demanding operation of stretching--SQ.

Sheet of DDQ quality should be used when drawing steel will not
provide a sufficient degree of ductility for fabrication of parts with
stringent drawing requirements, or applications that require the sheet
to be free from aging. This quality is produced by special steelmaking
and finishing practices. It is suitable for automotive front panels and
rear fenders.

Sheet of DQ quality has a greater degree of ductility and is more
consistent in performance than commercial steel, because of higher
standards in production, selection and melting of the steel. It is
suitable for automotive panels, audio-visual equipment, and heating
apparatuses.

Based on specification of LDR for classic deep-drawing steel, it is
possible to specify requirements for the volume fraction of martensite
F-M steel sheets as follows:

However, for the stress-strain states from uniaxial tension to
biaxial tension (stretching) are preferable to use IE and FLD. In terms
of suitability for stretching, materials with martensite precipitated in
the form of small islands are classified according to Fig. 16 as
follows:

Dual phase steel sheets represent progressive material, but
designers often do not know its advantages in comparison with classical
steel sheets. In the article, we described approach of predicting of
safety characteristics of auto body and technological formability from
dual phase steel sheets is based on the concept of producing steel
sheets "to measure for a specific auto body product" taking
into account the microstructure of ferritic-martensitic steel sheets,
mechanical properties and requirements of efficient economical
processing for a specific product.

Knowledge obtained at evaluation of formability of high-strength
micro-alloyed and dual-phase steels may be summarized as follows:

1. From this comparison one can see that dual phase steel have 42
and 135% higher values of strength and also higher values of deformation
work. In case of production of steel sheets by stretching with
deformation higher than 5% the increase of stress to 100 - 200 MPa
occurs.

2. Formability of high strength dual phase steels was compared to
formability of deep-drawn steel DC04. Deep drawing capacity steel DC 04
has better formability than dual phase steel, but differences were small
in some cases (material A, B, D).

3. Stretching capacity was compared with stretching capacity of
dual phase steel sheets with volume fraction of martensite lower than
25%. Dual phase steel sheets with fine ferrite-martensitic structure
with martensite precipitated in form of small islands in grains ferrite
boundaries have higher values of strength and plastic properties as
steel with martensite precipitated in form of bigger islands.

4. The measured results indicate that it is appropriate to use the
Keeler and Brazier empirical relation for prediction of critical values
of deformation.

5. Formability of dual-phase ferritic-martensite steels may not be
evaluated only on the basis of comparison of mechanical properties
values required at conventional steel sheets--Tab. 1. For comparison, on
the basis of limit drawing ratio there were determined conditions for
quality deep drawings (CQ, DQ, DDQ, EDDQ) on volume fraction of
martensite in structure and in the same similarly also for stretching
capacity (CSQ, SQ, HSQ, ESQ).

5. Acknowledgements

This work is a part of research project VEGA 1/0824/12 "Study
of formability aspects of coated steels sheets and tailored blanks"
supported by Scientific Grant Agency of the Ministry of Education,
Science and Research of Slovakia.

Authors also express their thanks for project APVV-0273-12
"Supporting innovations of autobody components from the steel sheet
blanks oriented to the safety, the ecology and the car weight reduction
" supported by Slovak Research and Development Agency.